Curable coating compositions have wide application in a number of fields, as they are useful for protecting various kinds of surfaces from severe chemical and physical environments. A solder mask, for example, is a permanent coating for a printed circuit board that must not cover certain parts of the circuitry on the board such as the contact points. A photoimageable, UV-curable solder mask is applied as a coating to the entire board, and then exposed to ultraviolet light through a pattern, or image. In the case of "negative resists," where the light falls on the coating, the coating hardens, or cures. The portions that are not exposed to the light remain unhardened and are washed away in a developer solution. Sometimes it is desirable to further treat the solder mask to toughen it. In that case, the coating might be baked in order to crosslink it and further improve its chemical, heat, and moisture resistance.
Materials which are useful in the above-described processes are well known, and are available as formulations that are developable in solvents made of volatile organic compounds (VOCs). VOCs, however, have been identified as pollutants and have been targeted by a variety of regulations aimed at reducing their use. Accordingly, efforts have been directed toward obtaining photoimageable coating materials that can be developed in aqueous solutions.
One technique for rendering a photoimageable coating material developable in an aqueous solution is to add acid functional groups to an organic molecule that is otherwise known to yield good physical properties when formulated as a coating. For example, certain commercially available formulations use polymers modified to incorporate significant amounts of carboxylic acid functional groups in order to confer dispersibility in aqueous base on an otherwise hydrophobic molecule. Such coatings are typically developable in alkaline water (for example, 1% carbonate) solutions. Of these, systems utilizing acrylates and epoxies predominate.
Acrylate/epoxy systems are well-known. The photoinitiated radical chain crosslinking reactions of acrylates represent the most common type of mechanism utilized in UV curable coatings. There are, however, inherent weaknesses in these systems. The radical chain reaction is inhibited by atmospheric oxygen, a problem that can be minimized through a combination of approaches such as using high intensity UV sources, higher levels of photoinitiators or synergists than would otherwise be required, or photocuring in vacuo or under inert atmospheres. In spite of these measures, there is usually an "incubation period" during which oxygen and photoinitiator are being consumed without significant crosslinking taking place. In solder masks, all of these approaches are frequently applied in an effort to reduce the necessary exposure times and provide a better surface cure. However, these efforts tend to have a negative impact on other performance features, such as sidewall profiles, thereby limiting image resolution. Additionally, acrylate based systems by definition have high levels of ester linkages, which can become hydrolytically weak links in demanding applications like the solder mask, where extremes of pH and temperature are encountered not only in process (such as plating baths) but sometimes in the finished product. Acrylate systems occasionally exhibit insufficient adhesion. Adhesion promoters can sometimes successfully be used, but these may interfere with other desirable properties of the coating. Finally, acrylate crosslinked coatings can suffer from brittleness due to shrinkage that occurs during photocuring, however this effect can be moderated by utilizing a thermal epoxy cure.
Curable systems that utilize the reaction of thiols with unsaturated hydrocarbons are also known. U.S. Pat. No. 4,020,233, issued to Morgan Apr. 26, 1977 relates to heat-activated compositions comprising ethylenically unsaturated compounds in combination with polythiols and pinacol catalysts. U.S. Pat. No. 4,006,270 issued to Morgan Feb. 1, 1977 relates to novel solid polyfunctional unsaturated compounds derived from styrene-allyl alcohol copolymers reacted with polythiols for use in imaging applications. U.S. Pat. No. 3,904,499 issued to Morgan Sep. 9, 1975 teaches the use of solid polythiols based on styrene-allyl alcohol copolymers with liquid polyfunctional unsaturated compounds. All of the above references teach the use of thiols which will yield ester-containing cross-links upon curing, and all use organic solvents in the development process.
Other reactions utilizing thiols and unsaturated compounds are known. U.S. Pat. No. 4,135,047 issued to Morgan Jan. 16, 1979 relates of the preparation of thioethers using a benzopinacol initiator. Acid-containing thiols are mentioned as potential raw materials. No specific use of the resulting material is disclosed, and there is no discussion of photoimaging, much less water development.
Thiols which are ester-free are known. See U.S. Pat. No. 4,591,522 issued to Kang, et al. May 27, 1986, which relates to liquid photopolymer formulations made with the reaction product of a non-ester containing polythiol with an unsaturated chlorendate to yield a fire-retardant, hydrolysis-resistant final product. Neither photoimaging nor water-developable formulations are disclosed.
Acid functionalized polythiols have been used in photoresists as adhesion promoters. See U.S. Pat. No. 5,091,483 issued to Lin et al. May 28, 1991. In this case, the polythiol is used as an adhesion-promoting additive at very low, non-stoichiometric amounts, in an attempt to avoid scumming in an acrylate curing system. No mention is made of thiol-ene photoimaging systems.
Acid functional polythiols of a different class, the .beta.-hydroxy-functional thiols, have been found to be useful in photoresist applications, and are disclosed in U.S. patent application Ser. No. 08/879,089 filed concurrently herewith by the same inventors.